この川の流れは急だ。

this (something or someone close to the speaker (including the speaker), or ideas expressed by the speaker)

river; stream; the .... river; (suffix used with the names of rivers)

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

stream; current; flow; course of events

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

urgent; sudden; abrupt; sharp; steep; precipitous; rapid; swift; fast; emergency; crisis; hurrying; haste; (in gagaku or noh) end of a song

be; is; indicates past or completed action; indicates light imperative